Dimensión Económica

The reasons for the large variation in fracture risk globally are largely unknown, however it has been suggested that environmental factors are of greater significance than genetic. Kanis et al.

(2012) comment on the usefulness of migration studies to determine the impact of environment on bone health, with examples of ‘the higher fracture probabilities among Chinese living in Hong Kong and Singapore compared with mainland China’, and the Japanese population of Hawaii compared to those in Japan. In addition to the previously discussed study of African migrants to the USA (Section 0), where the study population had migrated from an African country to a western country, there are several other possible migration study designs. For example, it is possible to compare rural-to-urban migrants with their rural counterparts, or to compare them to the urban-born population. However, there are very few published studies of rural-to-urban migration and bone health and none in sub-Sahara Africa.

In 2003, a study was conducted to determine prevalence of osteoporosis and fracture incidence in 1764 postmenopausal women who migrated from southern Italy to live in Milan, northern Italy (Varenna et al., 2003). It is not a rural-to-urban migration study per se, but the authors comment that the environment and lifestyle of southern Italy differs significantly from northern. To be classified as migrants, participants had to have migrated more than 20 years ago and lived continuously in Milan for at least 15 years. They were compared with 4018 Milanese women of similar age. Bone mineral status was assessed at the lumbar spine using DXA.

The migrant population was found to have a lower lumber spine BMD and a higher prevalence of osteoporosis. The migrant group had a higher BMI, however this was not associated with higher

101 | P a g e BMD as might have been expected based on other studies, which have suggested a positive relationship between BMI and BMD (Lloyd et al., 2014, Ho and Kung, 2005). Moreover, these findings were not as expected from previously published data which indicated a lower prevalence of osteoporosis and mortality due to hip fracture in southern Italy (Heyse et al., 1990). The authors suggest that perhaps like the study in Gran Canaria that socio economic status could be a factor influencing their findings between groups.

25 OH vitamin D was not measured in this study, and the authors recognise this as a limitation, especially as the migrant group from southern Italy are likely to have a darker skin pigment and this along with reduced sunshine associated with Milan’s more northerly latitude could have led to a lower 25 OH D status in the migrants. The calcium intake, based on a semi-quantitative questionnaire included only three dairy products (milk, yogurt and cheese), was lower in the migrants (p<0.01) and if this was combined with a lower vitamin D status due to reduced skin synthesis then this could have resulted in reduced absorption and bioavailability of calcium and potentially lead to poorer mineralisation.

Finally, the most recent study investigating the impact of rural-to-urban migration on bone mineral status (aBMD) was conducted in Hyderabad, India (Viljakainen et al., 2015). Skeletal sites measured included the lumbar spine and total hip using DXA. The study design was based on a comparison of a sibling-pair (n=185 sib-pairs). After adjusting aBMD for height, gender, age, and occupation, rural-to-urban migration was associated with higher BMDSPINE andBMDHIP. The authors suggest that ‘differences in lean mass and to a lesser extent fat mass, largely explained the BMD differences’ they observed. These findings are cross-sectional, and it is unknown whether this increased BMD, mainly due to differences in body composition, will translate into increasing fracture rate in later years.

S UMMARY

This section has highlighted interesting and apparent negative associations between urbanisation and bone health primarily defined in terms of higher fracture incidence and lower bone mineral status in HICs. Trends worldwide indicate the rapid and recent nature of the rise in fragility fracture incidence in many countries. This review has also underlined the complex myriad factors that affect bone health and their disparity within and across countries. Due to the paucity of data in sub-Saharan Africa, most of the literature is from non-African populations. However, it seems that potentially detrimental trends are beginning to emerge in some African countries, with higher fracture rates reported in regional studies in Tanzania and South Africa. With the predicted expansion of older populations across the continent, an increase in fracture incidence is likely;

however, the implications of the nutrition transition combined with significant levels of rural-to-urban migration are not well understood in this context, and to date no studies have been conducted in The Gambia.

Literature reviewed in Chapter 2 highlighted the available evidence suggesting that transition is occurring in The Gambia. Many aspects of rural lifestyle appear to be protective for bone health, but the impact of transition on bone health in The Gambia is unknown. The evidence suggests that the nutrition transition, associated with a more western diet and lifestyle are likely to be key contributory factors. The associated shift in body composition is likely to be important, along with the increasing prevalence of other NCDs.

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4 S ETTING AND D ^{ESIGN OF}

W ^OMEN ’ ^S M ^IGRATION S ^TUDY

This section comprises study objectives and hypotheses, the overall Women’s Migration Study (WMS) design, and details outlining the two study locations. Information about study setup and logistics are also included.